Fig. 277.—A Short Bridge.
Fig. 278.—A Bolt Truss.
[Figure 277] is a more improved style of bridge, the truss serving both to support the structure, and as a parapet. The top railing is of the same width as the sill, about one foot. The lower side may be cut away, giving the bridge a more finished appearance. The railing at the center is six inches thick, and three inches at the ends. The tie, h, is full width and four inches thick. A bridge of this kind will answer for heavy traffic, even if twenty feet in length. The bolt truss, in [figure 278], is adapted for a span of twenty-five feet. This makes a bridge of great firmness. Each set of truss-rods support a cross-sill. The road planks are laid crosswise of the bridge. The middle sills are sometimes half an inch lower than those along the sides, and should be four or five in number. The ends of the planks fit closely against the inside of the truss sills, thereby keeping the planks securely in place.
Fig. 279.—Bridge Braced From Below.
A common method of bracing is from below as shown in [figure 279]. This is not usually a good practice, as the braces are liable to be carried away by ice or floods.
ABUTMENTS, PIERS AND RAILINGS.
Fig. 280.—End Of A Bridge.